The present invention relates to a continuous process for the work-up of crude nitrobenzene. In this process, the crude nitrobenzene is washed in an acidic, in an alkaline and in a neutral wash consecutively. In the neutral wash, the organic phase containing nitrobenzene is first dispersed, then broken down in an electrophoresis unit and then the aqueous and organic phases are separated.
The prior art for neutral washing of crude nitrobenzene is dispersion of the organic phase with washing water using a mixing vessel or static mixer. The subsequent separation of the phases is conducted in separator vessels (settlers with or without coalescers) with the possible aid of demulsifiers (separating aids).
The basic electrophoresis process is described in DE-A-2 808 225. In this known process, an emulsion of nitrobenzene dispersed in waste sulfuric acid is produced in a batch process by stirring or recirculating, and this emulsion is then broken down in an electrode chamber.
DE-A-2808225 describes the workup of nitrobenzene from which excess benzene has already been separated. However, DE-A-2 808 225 does not disclose that the crude nitrobenzene contaminated with benzene has to be subjected to an acidic and then an alkaline wash followed by a neutral wash before the electrophoretic breaking down of the dispersion. Nor does this disclosure teach a separation of water and benzene to obtain higher purity of the nitrobenzene produced after the electrophoretic breaking down of the dispersion. Moreover, the process disclosed in DE-A-2 808 225 cannot be directly transferred to the purification of crude nitrobenzene by a neutral wash following the acidic and alkaline washes because, in the stages preceding the neutral wash, essential components affecting the result of the electrophoresis, such as sulfuric acid, are already washed out of the crude nitrobenzene.